Method and apparatus for injection molding of plastic materials

ABSTRACT

Injection molding of plastic materials by an apparatus comprising at least one injector including a valve pin displaceable in a controlled manner between a closed position and an open position by an electric rotary actuator. At least one step is provided of detecting the value of the torque developed by the electric rotary actuator when the valve pin is in the path from the closed position to the open position, and the torque value detected is used to pilot the molding process.

FIELD OF THE INVENTION

The present invention relates to injection molding of plastic materialsand particularly relates to a method of injection molding by means of anapparatus comprising a distributor of the fluid plastic material underpressure connected to at least one injector including a valve pindisplaceable between a closed position and an open position.

Typically, these injection molding methods comprise a step of fillingthe mold cavity with the plastic material, following displacement of thevalve pin from the closed position to the open position, followed by apacking step under pressure of the plastic material injected into thecavity, in which the valve pin is kept in the open position. The valvepin is then displaced from the open position to the closed position and,after a waiting time to allow the plastic material to solidify, themolded article is extracted from the mold.

The displacements of the valve pin of the or each injector aretraditionally operated by means of a fluid actuator. Recentlyapparatuses have been proposed in which the valve pin is driven by anelectric rotary actuator, in a controlled manner, by means of electronicsystems.

STATE OF THE PRIOR ART

The use of electric actuators for driving the valve pin of the or eachinjector of the molding apparatus is described, for example, indocuments JP-06114887, U.S. Pat. No. 7,121,820 and EP-2,679,374 (by thesame Applicant). The electric actuators, compared with fluid actuators,are much more easily controllable with the aid of electronic systems,which operate on the basis of process parameters, detected byappropriate sensors, and/or by means of specific algorithms. In thisway, it is possible to operate an efficient control of both the positionof the valve pin between the closed position and the open position tovary the flow of the injected plastic material during the molding cycle,as described in document U.S. Pat. No. 6,294,122, and the speed ofdisplacement of the valve pin during its movement from the closedposition to the open position. Thus, the documents WO-2012/074879 andWO-2012/087491 provide for the control of the electric actuator todisplace the valve pin in a continuous manner from the closed positionto the open position at an initial speed, and then to one or more higherspeeds compared to the initial one. This control is operated as afunction of the time or the space covered by the valve pin starting fromits closed position.

Controlling the position of an electrically-actuated valve pin is alsoknown from JP-2009298091A, JP-2002059467A. The publication “BRAUN P.:“PREAZISE POSITIONIERN”, Plastverarbeiter, Huethig GmbH, Heildelberg DE,1 Dec. 2005, pages 38-39, XP001155127, discloses the valve pin beingcontrollably operated by an electrical actuator and a torque is detectedand employed for providing a malfunction detection.

This type of control is difficult to be correlated to the actual processconditions, or rather to a series of strongly variable parameters, forexample, as a function of the changes in working conditions and thephysical state of the plastic material (temperature, viscosity, packingconditions within the mold) etc.

A more accurate control would be particularly desirable for theinjection molding of articles for which a high quality is required, bothfrom a mechanical and an aesthetic point of view, for example, in thecase of large motor vehicle components such as bumpers and the like. Inthis case, the molding apparatus typically includes a certain number ofinjectors distributed in the different zones of the mold: in the case inwhich these injectors are fed by a single distributor, appreciabledifferences in pressure and density of the injected plastic material canbe generated between the zones of the cavity next to the centralinjector(s) and those corresponding to the peripheral injectors. Forthis reason, in these cases, a sequential injection is used, in whichthe filling step of the mold cavity is in fact carried out bysequentially controlling the opening of the valve pins of the differentinjectors so as to progressively fill the mold cavity, starting from thecentral zone towards the peripheral zones. In this case, the valve pinsof the central injectors can remain open during the entire injectioncycle, or can be displaced to the closed position following the openingof the valve pins of the peripheral injectors. In this way, advancing ofthe filling front of the mold cavity is improved, and the localizedpressure caused by the accumulation of the plastic material in theinitially-filled zones of the cavity is reduced.

Particularly in the case of this type of sequential injection, thefilling step and the subsequent packing step must be well balancedbetween the various injectors and the different zones of the cavity, asotherwise, deformation problems of the molded piece could be generatedas well as tensionings with consequent reduction of the mechanicalproperties, localized increases in thickness, and in some casesfilaments and breakages during the molding step.

Similar problems, albeit minor, can also occur in injection molding ofarticles whose dimensions do not necessarily require a sequentialmethodology.

SUMMARY OF THE INVENTION

The object of the invention is to make an efficient solution to theabove technical problems available, through a control of the injectionprocess that is more directly correlated to the actual processparameters as well as to their variability.

In view of achieving this object, the invention relates to a method forinjection molding of the type defined at the beginning, whose uniquecharacteristic lies in the fact that it provides at least one detectingstep of the value of the torque developed by the electric rotaryactuator when the valve pin is in the path from said closed position tosaid open position, the torque value detected being employed to pilotthe molding process.

The invention is based on the finding that the torque developed by theelectric motor, which is required to move the valve pin, and alsopossibly to keep it temporarily stationary along the opening path, is afunction of the pressure of the plastic material injected into the mold.Through the detected torque value, the control of the electric actuatorcan, for example, act on the positioning of the valve pin to increase orreduce the flow of plastic material into the mold, or act on its speed,varying it in a suitable manner.

The detection of the torque developed by the electric actuator derivingfrom forces acting on the valve pin can be carried out in a more preciseand accurate in static conditions, or rather with a non-moving valve. Tothis effect, the invention advantageously envisages that the movement ofthe valve pin from the closed position to the open position isdiscontinuous and includes at least one temporary stop step of the valvepin in which the detecting step of the torque value of the electricactuator is carried out. This stop step can be followed by a briefreturn step of the valve pin towards the closed position, for apredetermined amount. This is particularly useful given the fact that,because of the coupling clearances between the pin and the actuator, theposition of the latter may not be univocally determined: the inversionof the movement of the valve allows, after its arrest, detection of thetorque of the electric motor having recovered any possible clearance.

Since the torque value is not always directly proportional to theinjection pressure, it is convenient to proceed to a preliminary initialcalibration step in which the detection of the torque value isconveniently carried out during the course of a series of empty openingand closing cycles of the valve pin, i.e. without injecting the plasticmaterial into the mold.

The method according to the invention is particularly advantageous inprocesses of sequential injection, which has been previously mentioned:in this case, the value of torque developed by the electric rotaryactuator of the valve pin of each injector is detected, and the torquevalues detected are employed, for example, to make these torquesessentially uniform by means of movement variations of the valve pins.This allows an optimal distribution of the pressure inside the moldcavity to be obtained, and therefore, ultimately, a better quality ofthe articles molded as such.

According to another aspect of the invention, an apparatus for injectionmolding of plastic materials into a mold cavity is provided, comprisinga distributor of fluid plastic material under pressure connected to atleast one injector including a valve pin displaceable between a closedposition and an open position, wherein the valve pin is controllablyoperated by an electric rotary actuator, characterized in that it isconfigured to detect the value of the torque developed by the electricrotary actuator at least when the valve pin is in the path from saidclosed position to said open position, and to employ the detected torquevalue to pilot the molding process.

During the molding method, at the end of the opening step, the vale pinis kept stationary in a packing step of the plastic material injectedinto the mold, and then returns to the closed position. Preferably, theapparatus according to the invention is configured to also detect thevalue of the torque developed by the electric rotary actuator during thepacking step and/or during the return of the valve pin to the closedposition.

Conveniently, the apparatus according to the invention is alsoconfigured for performing the movement of the valve pin from the closedposition to the open position in a discontinuous manner, with at leastone temporary stop step in which said detecting step of the torque valueis carried out. This temporary stop step can be followed by a returnstep of the valve pin towards said closed position, for a predeterminedamount.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described in detail with reference to theattached drawings, provided purely by way of non-limiting example, inwhich:

FIG. 1 is a schematic view in partial cross-section showing a part of aconventional injection molding apparatus,

FIG. 2 is a normalized diagram which shows the graph of the stroke ofthe valve pin, of the injection pressure and the torque developed by theelectric actuator of the molding apparatus of FIG. 1 during an injectioncycle, as a function of the time,

FIG. 3 is a diagram analogous to that of FIG. 2 in which the speed ofthe valve pin has been added to the graph,

FIG. 4 is a diagram analogous to that of FIG. 3 relative to the case inwhich a stop step of the valve pin during its opening displacement isenvisaged,

FIG. 5 is a schematic elevation view showing a configuration of themolding apparatus suitable for carrying out a cycle of sequentialinjection,

FIG. 6 is a diagram analogous to that of FIG. 2 referring to the moldingapparatus of FIG. 5, and

FIG. 7 is a diagram analogous to that of FIG. 3 still referring to themolding apparatus of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows an apparatus for injection molding of plasticmaterials comprising, in a generally conventional manner, an injector 3connected to a distributor of the melted plastic material or hot chamber2 and comprising a nozzle 5 provided at its free end with a nozzleterminal 6 in communication with the cavity of a mold S through aninjection passage (“gate”). The flow of the plastic material through thenozzle terminal 6 is controlled by a valve pin 7 axially displaceablealong the nozzle 5 by an actuator 8, between a lowered closed positionshown in FIG. 1, and a raised open position.

The actuator 8 is an electric actuator, and more specifically a rotaryelectric motor: its arrangement, represented in FIG. 1, is purelyindicative, as it is conveniently of the type described and illustratedin the already cited document EP-2,679,374 by the same Applicant,according to which the rotary shaft of the electric motor 8 drives thevalve pin 7 through a reversible-type transmission, typically includinga screw and nut unit and a pivoting lever.

The electric motor 8 is operatively connected to an electronic controlunit 4 configured to drive the electric motor 8, and consequently thevalve pin 7, in a controlled manner as a function of various parametersincluding, as will be seen, the value of the torque developed by theelectric motor 8 during the displacement of the valve pin 7 andspecifically when it is in the path from the closed position to the openposition.

The invention envisages that the injection cycle of the plastic materialinto the cavity of the mold S is in fact also controlled i.e. piloted bymeans of the torque value, detected with systems within the scope ofthose skilled in the art, which is a function of the forces acting onthe valve pin 7.

FIG. 2 shows, in normalized diagram form, the correlation between thestroke of the valve pin (solid line A) from the closed position to theopen position, the trend of the injection pressure (the graph with thedashed line B) and that of the torque developed by the electric motor 8(graph with a solid line C), as a function of the time. Typically, theinjection cycle comprises a step of filling the mold cavity S with theplastic material, following the displacement of the valve pin 7 from theclosed position to the open position, followed by a packing step underpressure of the plastic material injected into the cavity, in which thevalve pin 7 is kept stationary in the open position. The valve pin 7 isthen made to return from the open position to the closed position.

As seen in the diagram of FIG. 2, the injection pressure graduallyincreases up to a maximum reached at the packing step, and thendecreases and remains essentially constant, and then zeroes when thevalve pin 7 returns to the closed position. The torque developed by theelectric motor 8 is firstly negative, at the beginning of the opening ofthe valve pin 7, and increases with the increase of the injectionpressure essentially following the trend, even at the packing step, atthe end of which it increases again to start the return of the valve pin7 towards the closed position, and then progressively decreases up to avalue sufficient to keep the valve pin 7 closed with the mold cavity Sfull and packed.

In the diagram of FIG. 3, the graphs A relative to the stroke of thevalve pin 7, B relative to the injection pressure and C relative to thetorque developed by the electric actuator 8 are represented, as afunction of time, in the case in which the speed of displacement of thevalve pin 7 from the closed position to the open position is notconstant, but includes an initial step at a first speed, followed by anintermediate step at a higher speed (and possibly a final step at aneven greater speed). The trend of the speed is represented in FIG. 3 bygraph D: as can be seen, the motion of the valve pin 7 from the closedposition to the open position is not continuous but is discontinuous, asat least one stop step is provided, shown in the graph of FIG. 3 by thestretch at zero speed between the initial stretches at a lower speed andthose at an intermediate speed or the final higher speed of graph D.This stop step of the valve pin 7 is considered appropriate andadvantageous for a more precise detection of the torque developed by theelectric motor 8 during the injection cycle: in fact, during themovement of the valve pin 7, the measured torque value is not easilycorrelatable to the injection pressure, as there are dynamic slidingeffects of the valve 7 on the relative valve guide, as well as friction,even at the level of the kinematic mechanisms, which has been mentionedabove, in the transmission between the electric motor 8 and the valve 7.It is therefore more appropriate to detect the torque when the valve pin7, and therefore the electric motor 8, are stationary for a period oftime that can also be very brief.

The diagram of FIG. 4 is analogous to that of FIG. 3 and differstherefrom by the fact that the trend of speed of the valve pin 7represented by the graph D shows, at the instant of stopping themovement of the valve pin 7 towards the open position, a brief reversalof motion in which there is a step of partial return towards the closedposition. This reversal, of a slight, predetermined amount,advantageously allows detection of the value of the torque developed bythe electric motor 8, also taking account the clearances present in therelative transmission kinematic mechanism, and in particular between theend of the valve pin 7 opposite to the nozzle terminal 6 and itsattachment to the transmission driven by electrical motor 8. Thisconnection, typically produced by means of a head of the valve 7 coupledwith a clearance generally in the order of 0.1-0.2 mm to a bayonet seat,can determine a non-univocal position of the valve 7 itself. In fact, ifduring the opening movement, the drag forces operated by the fluidplastic material on the valve 7 are high, this valve 7 will be closer tothe lower abutment of the bayonet seat relative to its head. The briefreturn movement of the valve pin 7 operated following its arrest thenallows recovery of the clearance mentioned above and obtainment of amore accurate torque measurement.

An analogous step of brief motion reversal of the valve pin 7 can alsobe provided at the end of its opening stroke, or rather at the beginningof the packing step of the plastic material into the mold cavity, for afurther detecting step of the torque developed by the electric motor 8.

The methodology described above with reference to the single injector 3is advantageously applicable to the case of sequential injectionoperated through a plurality of injectors arranged in a series with acentral injector 3 a, intermediate injectors 3 b, 3 c and end injectors3 d, driven by respective rotary electric motors 8 a, 8 b, 8 c, 8 d,controlled while allowing for the detection of the torque developedthereby.

In the case of the example represented in FIG. 5, the sequentialinjection molding apparatus is used for producing a large article withhigh aesthetic value, such as a bumper P for cars. The injectors 3 a-3 dare fed from a common distributor or hot chamber 2 and are positioned assuch: the first one at a central injection passage of the mold, and theothers at gradually more peripheral injection passages.

FIG. 6 is a normalized diagram that shows, as a function of the time,the stroke of the valve pins, respectively, of the central injector 3 a(graph Aa), the intermediate injectors 3 b and 3 c (graphs Ab and Ac)and the end injectors 3 d (graph Ad), the trend of the injectionpressure (graph B) and the trends of the torques, respectively, of motor8 a (graph Ca), motors 8 b (graph Cb), motors 8 c (graph Cc) and motors8 d (graph Cd). Detecting the torque values of the motors 8 a-8 dallows, for example, during the sequential injection, the definition ofa movement algorithm of the different valve pins 7, suitable foroptimizing the cycle. For example, during the injection cycle, thevarious valve pins can be moved to the open position in such a way sothat the torque of the motor detected for each injector has a valuesimilar to that of the previous injector.

Even in the maintenance step and/or in the closing step, it is usefuland appropriate to perform the detection of the torque absorbed by theelectric motors of the injectors: for example, it can be derived fromthe diagram of FIG. 7, which will be discussed, that the stroke of thevalve pin of the central injector Aa stops during closing, at a valueclose to 20% when the torque detected Ca of the respective electricmotor during this brief stop is greater than a limit value, determinedfor example as a function of injection cycles previously carried out.The torque control in the maintenance and/or closing step, in the secondcase following a brief temporary stop of each valve pin 7, may allow,for example, the standardization of the torques of the electricactuators of all the injectors in these steps, also by means of movementvariations of the relative valve pins, so as to more efficiently controlthe deformations of the molded pieces in the zones close to theinjectors.

In any case, the control also based on detecting the torques applied bythe electric motors allows a better balance of the pressures anddistributions of the material within the mold cavity, in such a way thatthe pressure and density of the plastic material can be more uniform. Inthis way, the piece P molded at the end of the sequential process willhave improved characteristics in terms of mechanical properties andaesthetic qualities.

The diagram represented in FIG. 7 also shows in this case, in terms ofnormalized values as a function of time, the trends of the strokes, theinjection pressures and the torques developed by the electric actuators8 a-8 d of the injectors 3 a-3 d, in the case in which a speed controlof the relative valve pins 7 is provided for. For each of them, atemporary stop step may be provided, possibly followed by a brief returnto the closed position, for a better detection of the torques.

The method according to the invention may conveniently include aninitial calibration step in which the detection of the torque value iscarried out during a series of empty opening and closing cycles of theor each valve pin 7, or rather without injection of plastic materialinto the mold S.

Below, the unique aspects of the molding method according to theinvention will be summarized, and the effects they cause.

-   -   A. The front pressure of the valve, and therefore immediately        upstream of the “gate”, is correlated with the torque necessary        to keep the valve in the open position or partially open        position (and vice versa: the torque is correlated to the        pressure).    -   B. This functionality is made possible by the fact that the        kinematic chain between the electric motor and the valve is        reversible.    -   C. If the valve is closed with position control, the detected        torque is proportional to the pressure of the plastic material        within the mold cavity at the injection point. The value is not        exact because, in addition to the various frictional forces,        there are phenomena of freezing of the plastic material on the        gate which retain the valve.    -   D. The torque detected during the movement, in particular during        an empty cycle, therefore without injection, but with plastic        material within the nozzle, can give an indication of the        viscosity of the material and its temperature. In particular,        during the initial calibration steps, if the torque value        exceeds the preset limits (as a function of the provided plastic        material) it means that the viscosity of the molten material is        not sufficient, and therefore the current temperature of the        nozzle of the hot chamber is not adequate to start molding.    -   E. During the initial calibrations, the opening and closing        limits (TDC and BDC) can be determined as a function of the        unique torque required for the movement, except that the valve        is free to move immersed in the molten plastic.    -   F. If during calibration, torque values envisaged for determined        strokes of the valve are not respected within appropriate        limits, the system generates errors: for example, if after        finding the TDC, the system detects the BDC (owing to the        increase in torque over a certain limit), before having carried        out the entire mechanical stroke planned for the project, minus        a tolerance, the system reports an error. An error can also be        reported in the case that, during the stroke at constant speed,        the torque oscillates or varies beyond certain limits,        indicating, for example, jamming of the valve on the valve        guide, and possible seizures, or abnormal mechanical breakages.    -   G. During the movements carried out during the molding cycle, it        is possible to set positions and torque limits (upper and lower)        to determine the correct functioning of the system and the        process. The process can continue limiting the torque to these        limited values, or can generate alerts or errors reported to the        operator, or as an electrical signal of a discarded piece to the        press or to the production island, to separate this piece from        the “good” ones.    -   H. During molding, and in particular during the startup cycles        (which can be from 10 to 30 cycles depending on the type of        process) due to the slow stabilization of the process,        variations in the working conditions can be reported, such as        the required torques for the movement, or the closure heights,        so that, for example, the plastic layer that is interposed        between the pin and the “gate” can vary (generally decreasing).        Another typical example is the heating of the mold, which        decreases the injection pressure, and therefore the detected        torques. The control system can adapt to the variation of the        working conditions, no longer defining the closure height as a        fixed position with respect to an absolute reference (e.g. TDC),        but as a function of the overcoming of a certain torque, always        within tolerance limits both of position and of torque.    -   I. The torque, both during the movement and when stationary, can        vary continuously from negative values to positive ones, thanks        to the functionality of the control software/firmware, in        positions implemented in the electronic control unit of the        electric motor. The torque is negative or positive as a function        of the reaction that it must exert on the valve to compensate        for the forces acting thereon: traction forces if there is a        dragging of the valve during the injection (in practice the        viscous or shear forces are high, for example, with viscous        materials and very long nozzles or very large valves, so that        the surface of the valve is large with respect to the annular        passage section of the molten plastic material, or they can be        compression or thrust forces, such as those exerted by the        pressure of the plastic material which presses on the front        projection of the valve (and therefore as a function of the        diameter of the valve and of the injection pressure).    -   J. Opening of the valve can occur at one or more different        speeds.    -   K. It is favorable and more efficient to detect the torque when        the actuator is temporarily stationary.    -   L. In any case, it is also possible to use the torque values        during movements, for example, if empty calibration cycles are        used, so that the values detected during these cycles are        subtracted or otherwise processed together with the graphs of        the torque detected during molding.    -   M. Due to the coupling clearances between the pin and the        actuator, the position of this may not be determined: indeed        during opening, if the drag forces are high, it is verified that        the valve will be closer to the lower abutment of the bayonet        seat of the valve head itself, while if the pressure forces are        high, the valve head will rest on the upper abutment of the        bayonet seat (by lower, it is intended closer to the chamber,        while higher means further away from the chamber). The same        occurs during the closing movement.    -   N. It is preferable during the opening step to interrupt the        movement for a brief moment to detect the torques and        consequently the forces acting on the valve.    -   O. If the torques are negative, and therefore the drag index        forces higher than the pressure forces, the valve will rest on        the lower abutment of the seat. To detect a correct pressure it        is therefore appropriate to slightly reverse the motion in order        to recover the clearance on the valve head.    -   P. The control software can use the torque values detected for        analysis or verification of the process or to set system        parameters via complex algorithms.    -   Q. In particular, during the sequential injection, it is        possible to define a movement algorithm as a function of the        torques detected by more actuators.    -   R. The method can analyze a single molding cycle, or a defined        series of cycles in order to dynamically vary the setting values        of each actuator and to reach the convergence with respect to a        desired result.    -   S. One objective can be that of obtaining equal final torques on        all or a determined number of actuators, as a function of the        molding and process characteristics. The system can close the        “gates”, by throttling the pressure transmitted downstream, so        that the torques result as being be within a certain range. This        range can be automatically set or determined as a function of        the results of the torque obtained during a process of standard        sequential injection (with opened and closed nozzles at maximum        speed and at a fully open height). In this case, it is possible        that the software determines the two offset ranges (preset or        percentage) relative to the average value of the torques.    -   T. During the sequential injection step, the software can open        the valves to the height necessary to maintain the relative        torque at a value similar to the preceding actuator.    -   U. The software can also close the preceding valves, if their        relative torque exceeds, during the injection step, the set        values, or is greater than the torque of the actuator that        follows it in the sequence. In fact, when the successive ones        begin to open, the preceding actuators can begin to close until        the target torque is reached. In this step, the actuator may        stop in order to detect the actual torque and to decide whether        to close more to further limit the torque and thus the pressure.    -   V. The end result is that of being able to produce a more        uniform redistribution of the pressures within the cavity of the        mold and therefore a better quality of the molded articles.

Of course, the embodiments of the invention may be varied widely withrespect to those described and illustrated, without departing from thescope of the present invention as defined by the following claims.

What is claimed is:
 1. An apparatus for injection molding of plasticmaterial into a mold cavity, comprising a distributor of fluid plasticmaterial under pressure connected to at least one injector including avalve pin displaceable between a closed position and an open position,wherein the valve pin is controllably operated by an electric rotaryactuator, said apparatus being configured to detect a value of thetorque developed by the electric rotary actuator at least when the valvepin is in a path from said closed position to said open position, and toemploy the detected torque value to pilot a molding process.
 2. Theapparatus according to claim 12, wherein at the end of the openingdisplacement, the valve pin is kept stationary during a packing step ofthe injected plastic material and then it returns to the closedposition, said apparatus being configured to also detect the value ofthe torque developed by the electric rotary actuator during said packingstep and/or during return of the valve pin to the closed position. 3.The apparatus according to claim 12, further configured to carry outdisplacement of the valve pin from the closed position to the openposition in a discontinuous fashion, with at least one temporary stopstep of said valve pin at which said torque value is detected.